xref: /openbmc/linux/arch/arm64/kernel/kaslr.c (revision fc062ad8)
1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * Copyright (C) 2016 Linaro Ltd <ard.biesheuvel@linaro.org>
4  */
5 
6 #include <linux/cache.h>
7 #include <linux/crc32.h>
8 #include <linux/init.h>
9 #include <linux/libfdt.h>
10 #include <linux/mm_types.h>
11 #include <linux/sched.h>
12 #include <linux/types.h>
13 #include <linux/pgtable.h>
14 #include <linux/random.h>
15 
16 #include <asm/cacheflush.h>
17 #include <asm/fixmap.h>
18 #include <asm/kernel-pgtable.h>
19 #include <asm/memory.h>
20 #include <asm/mmu.h>
21 #include <asm/sections.h>
22 #include <asm/setup.h>
23 
24 enum kaslr_status {
25 	KASLR_ENABLED,
26 	KASLR_DISABLED_CMDLINE,
27 	KASLR_DISABLED_NO_SEED,
28 	KASLR_DISABLED_FDT_REMAP,
29 };
30 
31 static enum kaslr_status __initdata kaslr_status;
32 u64 __ro_after_init module_alloc_base;
33 u16 __initdata memstart_offset_seed;
34 
35 static __init u64 get_kaslr_seed(void *fdt)
36 {
37 	int node, len;
38 	fdt64_t *prop;
39 	u64 ret;
40 
41 	node = fdt_path_offset(fdt, "/chosen");
42 	if (node < 0)
43 		return 0;
44 
45 	prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
46 	if (!prop || len != sizeof(u64))
47 		return 0;
48 
49 	ret = fdt64_to_cpu(*prop);
50 	*prop = 0;
51 	return ret;
52 }
53 
54 struct arm64_ftr_override kaslr_feature_override __initdata;
55 
56 /*
57  * This routine will be executed with the kernel mapped at its default virtual
58  * address, and if it returns successfully, the kernel will be remapped, and
59  * start_kernel() will be executed from a randomized virtual offset. The
60  * relocation will result in all absolute references (e.g., static variables
61  * containing function pointers) to be reinitialized, and zero-initialized
62  * .bss variables will be reset to 0.
63  */
64 u64 __init kaslr_early_init(void)
65 {
66 	void *fdt;
67 	u64 seed, offset, mask, module_range;
68 	unsigned long raw;
69 
70 	/*
71 	 * Set a reasonable default for module_alloc_base in case
72 	 * we end up running with module randomization disabled.
73 	 */
74 	module_alloc_base = (u64)_etext - MODULES_VSIZE;
75 	dcache_clean_inval_poc((unsigned long)&module_alloc_base,
76 			    (unsigned long)&module_alloc_base +
77 				    sizeof(module_alloc_base));
78 
79 	/*
80 	 * Try to map the FDT early. If this fails, we simply bail,
81 	 * and proceed with KASLR disabled. We will make another
82 	 * attempt at mapping the FDT in setup_machine()
83 	 */
84 	fdt = get_early_fdt_ptr();
85 	if (!fdt) {
86 		kaslr_status = KASLR_DISABLED_FDT_REMAP;
87 		return 0;
88 	}
89 
90 	/*
91 	 * Retrieve (and wipe) the seed from the FDT
92 	 */
93 	seed = get_kaslr_seed(fdt);
94 
95 	/*
96 	 * Check if 'nokaslr' appears on the command line, and
97 	 * return 0 if that is the case.
98 	 */
99 	if (kaslr_feature_override.val & kaslr_feature_override.mask & 0xf) {
100 		kaslr_status = KASLR_DISABLED_CMDLINE;
101 		return 0;
102 	}
103 
104 	/*
105 	 * Mix in any entropy obtainable architecturally if enabled
106 	 * and supported.
107 	 */
108 
109 	if (arch_get_random_seed_long_early(&raw))
110 		seed ^= raw;
111 
112 	if (!seed) {
113 		kaslr_status = KASLR_DISABLED_NO_SEED;
114 		return 0;
115 	}
116 
117 	/*
118 	 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
119 	 * kernel image offset from the seed. Let's place the kernel in the
120 	 * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
121 	 * the lower and upper quarters to avoid colliding with other
122 	 * allocations.
123 	 * Even if we could randomize at page granularity for 16k and 64k pages,
124 	 * let's always round to 2 MB so we don't interfere with the ability to
125 	 * map using contiguous PTEs
126 	 */
127 	mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1);
128 	offset = BIT(VA_BITS_MIN - 3) + (seed & mask);
129 
130 	/* use the top 16 bits to randomize the linear region */
131 	memstart_offset_seed = seed >> 48;
132 
133 	if (!IS_ENABLED(CONFIG_KASAN_VMALLOC) &&
134 	    (IS_ENABLED(CONFIG_KASAN_GENERIC) ||
135 	     IS_ENABLED(CONFIG_KASAN_SW_TAGS)))
136 		/*
137 		 * KASAN without KASAN_VMALLOC does not expect the module region
138 		 * to intersect the vmalloc region, since shadow memory is
139 		 * allocated for each module at load time, whereas the vmalloc
140 		 * region is shadowed by KASAN zero pages. So keep modules
141 		 * out of the vmalloc region if KASAN is enabled without
142 		 * KASAN_VMALLOC, and put the kernel well within 4 GB of the
143 		 * module region.
144 		 */
145 		return offset % SZ_2G;
146 
147 	if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
148 		/*
149 		 * Randomize the module region over a 2 GB window covering the
150 		 * kernel. This reduces the risk of modules leaking information
151 		 * about the address of the kernel itself, but results in
152 		 * branches between modules and the core kernel that are
153 		 * resolved via PLTs. (Branches between modules will be
154 		 * resolved normally.)
155 		 */
156 		module_range = SZ_2G - (u64)(_end - _stext);
157 		module_alloc_base = max((u64)_end + offset - SZ_2G,
158 					(u64)MODULES_VADDR);
159 	} else {
160 		/*
161 		 * Randomize the module region by setting module_alloc_base to
162 		 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
163 		 * _stext) . This guarantees that the resulting region still
164 		 * covers [_stext, _etext], and that all relative branches can
165 		 * be resolved without veneers.
166 		 */
167 		module_range = MODULES_VSIZE - (u64)(_etext - _stext);
168 		module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
169 	}
170 
171 	/* use the lower 21 bits to randomize the base of the module region */
172 	module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
173 	module_alloc_base &= PAGE_MASK;
174 
175 	dcache_clean_inval_poc((unsigned long)&module_alloc_base,
176 			    (unsigned long)&module_alloc_base +
177 				    sizeof(module_alloc_base));
178 	dcache_clean_inval_poc((unsigned long)&memstart_offset_seed,
179 			    (unsigned long)&memstart_offset_seed +
180 				    sizeof(memstart_offset_seed));
181 
182 	return offset;
183 }
184 
185 static int __init kaslr_init(void)
186 {
187 	switch (kaslr_status) {
188 	case KASLR_ENABLED:
189 		pr_info("KASLR enabled\n");
190 		break;
191 	case KASLR_DISABLED_CMDLINE:
192 		pr_info("KASLR disabled on command line\n");
193 		break;
194 	case KASLR_DISABLED_NO_SEED:
195 		pr_warn("KASLR disabled due to lack of seed\n");
196 		break;
197 	case KASLR_DISABLED_FDT_REMAP:
198 		pr_warn("KASLR disabled due to FDT remapping failure\n");
199 		break;
200 	}
201 
202 	return 0;
203 }
204 core_initcall(kaslr_init)
205